Pilot-assisted pressure relief valves, often called overcenter or holding valves, are used to control fluid flow to and from an actuator and to hydraulically lock the actuator in position when fluid flow is terminated. The valve prevents load-induced "runaway" and provides a static overload relief function Some of such valves have also included a check valve which allows flow to proceed unimpeded from the source to the actuator, but prevents fluid flow from the actuator to the source.
One common application for valves of this type is the control of fluid flow to an actuator used for operating a boom assembly. In order to raise the boom assembly, fluid is directed to the boom actuator via the check valve section of the pilot-assisted relief valve. As soon as flow terminates, the check valve operates to prevent return fluid flow from the actuator to the source and in this way the load is locked in position.
Examples of such pilot-assisted pressure relief valves include the valves disclosed in U.S. Pat. Nos. 4,336,826 and 4,346,733, which are assigned to the assignee of the instant invention.
Valves of this general type include a valve body defining an internal bore that slidably supports an elongate piston. A spring-loaded valve seat is mounted at one end of the valve body bore and is engagable with the piston. The piston and valve seat cooperate to control fluid flow between axially-spaced sets of radial ports which are formed in the valve body.
A portion of the end of the piston which engages the valve seat is exposed to fluid pressure present in one set of ports. Such pressure develops a force on the piston which urges it toward an open position, away from the valve seat. An adjustable principal spring which is at the opposite end of the valve body opposes this fluid force and maintains piston closure until the fluid force exceeds the spring force. Thus, adjustment of the spring determines the relief setting of the valve. Should an excessively high or overload pressure be encountered, the piston will move from its valve seat and allow fluid flow to the source until the pressure is reduced below the relief setting.
Such pressure relief valves have a pilot-assist feature to exert a force, on a pilot pressure area, in opposition to the principal spring. Such pilot-assist may be in various forms. Regardless of its exact form, the pilot-assist allows application of a pilot pressure from a pilot pressure passage formed in the valve body. The applied pilot pressure exerts a force on the principal piston in opposition to the spring force.
By selectively applying the pilot pressure to the pilot pressure area, controlled opening of the valve can be achieved to allow fluid flow from the actuator to the source, and thus effect lowering of the boom assembly. If the pilot pressure flow is terminated, the piston will immediately reclose and prevent further flow from the actuator. Should the load begin to "run away," the principal piston will throttle or terminate the flow due to the reduced force on the effective pressure area defined on the valve engaging end of the piston
While pilot-assisted pressure control valves of the type described have been generally acceptable for a variety of tasks, there are a number of problems and shortcomings the solution of which has led to this invention.
In particular, in some cases oil pressure in the valve body in the space around the spring has caused backpressure causing specific problems. For one thing, because of such backpressure the main piston is sometimes too slow in its reaction to pressure impulses The result is a slower-than-desirable relief of pressure Such backpressure also makes accurate setting of threshold pressures difficult or impossible. Prior pilot-assisted pressure relief valve designs did not allow solution of such problems in a simple manner, particularly in a valve of very compact construction.